This invention relates generally to a pneumatic transmitter operating in conjunction with a magnetic follower associated with a variable-area flowmeter to produce a pneumatic signal which is a function of flow rate, and more particularly to a compensated pneumatic transmitter producing a pneumatic signal which accurately reflects flow rate.
In a variable area meter for measuring flow rate, only the orifice area is varied as a function of flow, a constant pressure difference being maintained. The typical variable area meter is constituted by an upright, tapered tube containing a weighted float which is raised to a position of equilibrium between the downward force of the float and the upward force of the fluid flowing past the float through the surrounding annular orifice. The flow restriction is the area of the annular orifice, this area being enlarged as the float rises in the tapered tube. The pressure differential is fixed, this being determined by the weight of the float and the buoyant forces.
In many flowmeter applications it is necessary to translate the changing vertical position of the float within the flow tube into a corresponding external indication. Where, for example, the process fluid is opaque, the float cannot be seen through the transparent tube, hence direct visual indication along the tube is precluded. In other situations, the flow tube must be made of metal or other opaque material, so that it is then necessary to provide means for registering the float position at a point external to the tube.
Mechanical coupling of the float to an external indicator is generally unsatisfactory, since frictional and other loads thereby imposed on the float interfere with its free motion and distort the flow rate reading. In order to minimize loading the float, it is known to use a magnetic, nonmechanical coupling between the movable float and an external indicator.
Thus both the Conkling U.S. Pat. No. 3,315,523 and the Busillo U.S. Pat. No. 3,164,989 disclose arrangements in which the float in the variable tube is provided with a guided extension rod to which a bar magnet is attached, the bar magnet moving up and down in accordance with changes in the vertical float position. The bar magnet cooperates with a rotatable follower magnet mounted on a shaft to which an indicating pointer is attached. The follower magnet is caused to rotate in a direction and to an extent which is a function of the bar magnet movement and hence of flow rate.
The difficulty experienced with magnetic followers of the type disclosed in the Conkling and Busillo patents is that when the hydraulic characteristics of the flowmeter are aligned to the follower output by small manipulations of zero and span adjustment, the movement of the float from its minimum to its maximum position is accompanied by an indicator movement having a bowed characteristic that is essentially symmetrical with respect to the 50% flow rate position. In other words, though the lift of the float in the tube has a straight-line relationship to the indication, the float lift exhibits a non-linear or curved relationship to flow rate which takes the form of a bow-shaped curved extending between the extremities of float travel.
In the copending U.S. Pat. application of Metzger, Ser. No. 564,668, filed Apr. 3, 1975, also assigned to Fischer and Porter Company, the assignee herein, there is disclosed a magnetic follower arrangement for a variable area flowmeter which includes simple correction means adapted to bring about a linear relationship between the flow rate and the output indication independently of the adjustment of zero and span. In the Metzger arrangement, the vertical movement of a float in a variable area flow tube is converted by a magnetic follower into rotary motion for operating a pointer along a scale, the follower being compensated for by an eccentric mass to effect the desired linear relationship.
In the Metzger arrangement, the reading of the variable area flowmeter is produced by a pointer which rotates with respect to a circular scale. However, there are many situations in industrial process systems where it is necessary to convert the reading of a meter into a corresponding pneumatic signal suitable for transmission to a remote station for operating indicator or control mechanisms.